Local Area Networks (LAN's) that transmit energy in both directions along a transmission medium (a bus) have been used to signal at transmission rates up to about 10 Mb/s. Various schemes such as CSMA/CD (Metcalfe, R. M. and Boggs, D. R., "Ethernet: Distributed Packet Switching for Local Computer Networks", Commun. ACM, Vol. 19, No. 7, pp 395-403, 1976) and token-passing, are used to control access to the medium by stations wishing to transmit. At higher transmission speeds, above about 10 Mb/s, these types of schemes become increasingly inefficient. Methods in which one or more buses are used that transmit energy in only one direction called UBS's (uni-directional bus systems) were then invented to improve efficiency at high speeds. One scheme, Fasnet, described in U.S. Pat. No. 4,532,626, uses two buses, one to carry traffic in one direction and a second, parallel bus to carry traffic in the other direction. A variation called Expressnet (Tobagi, F., Borgonovo, F. and Fratta, L., "Expressnet: A High-Performance Integrated-Services Local Area Network", IEEE J. on Selected Area in Communications, Vol. SAC-1, No. 5, pp 893-913, November 1983) folds a single bus past all stations once, back to the first station and then past all stations a second time in the same direction.
UBS's are much more efficient than the bi-directional bus schemes. They invariably operate in "cycles". An event of some type starts a cycle in which all stations with traffic to transmit send a given number of packets of information. The end of a cycle occurs when one or more stations observes that activity on the medium has ceased. A new cycle is then started. However, it takes of the order of one round-trip propagation time before all stations can determine that activity has ceased and that a new cycle should start. This period is usually small relative to the. length of a cycle. In a typical example, a cycle might consist of 100 packets, each of 100 .mu.s duration (e.g. 1000 bit packets at a signalling rate of 10 Mb/s). The round trip delay time might be 25 .mu.s(e.g. 5 km round-trip length .times.5 .mu.s/km). Thus, the length of cycle would be 100.times.100 .mu.s=10 ms while the "wasted" round trip delay of 25 .mu.s would be negligible in comparison.
However, consider a MAN (Metropolitan Area Network) application where the signalling rate is 1 Gb/s, the length of the network is 50 km round-trip and packets are short, say 250 bits in length. The 100 packet cycle now takes 25 .mu.s while the round-trip delay is approximately 250 .mu.s. The efficiency (utilization) of such a system would have dropped from 99.7% in the previous example to 9.1%.
The same type of inefficiency is experienced in other types of LAN's operating at high speed. A token-passing ring, for example, ensures fair access for all stations by circulating a token from station-to-station around the ring. When a station has the token it may send up to a given number of packets before it must relinquish the token by passing it to the next station. A cycle is now the time it takes for the token to pass once around the ring; the "wasted" round trip delay is now the time for the token to physically propagate from station to station and to be processed by each station. Thus, the performance of a ring at high speed is limited by the same factors as limit UBS's.
A variation of the token ring scheme is a slotted ring in which a slot is seized by the station wishing to transmit by having the station set a BUSY bit to logic 1 (Wilkes, M. V. and Wheeler, D. J., "The Cambridge Digital Communications Ring", Proc. of the local area communication network symposium, Boston, Mass., USA, 1979). The slot is freed by either the origination or destination station resetting the BUSY bit to logic 0. The slot may be used more than once as it propagates around the ring in the case where the destination station rests the BUSY bit. Fairness is guaranteed by different mechanisms in different systems. In ORWELL (Adams, J. L. and Falconer, R. M., "Orwell: A Protocol for Carrying Integrated Services on a Digital Communications Ring", Electronics Letters, Vol. 20, No. 23, pp 970-971, 8th November 1984) and FDDI (American National Standard, FDDI Token Ring, Media Access Control Draft Proposal document X3T9.5/83-16, Rev. 8, 1 March 1985) the time taken for a token to pass around the loop is used to control access to the loop.